FPGA hello world demos seem to focus on blinking an LED. Since the fomu has a usb port and the D+/D- pins are directly exposed it makes sense to use this as a USB serial port (aka CDC/ACM) and print "hello world" and control the led from the serial port.
This repo contains two such demos. For building these you need the yosys/nextpnr/icestorm toolchain, futhermore you need the tinyfpga usbserial from David Williams (this is a git submodule of this repo, so just clone it recursively). For loading this on the fomu you need dfu-util.
This prints helloworld onto the USB serial port, on linux you can enjoy this by connecting to /dev/ttyACM0
For loading this on your fomu and connecting to it just run: make hw-load && screen /dev/ttyACM0
This demo allows you to connect to the fomu on /dev/ttyACM0 and controlling the colors by send the chars r,g or b, any other character shuts off the LED, you also get feedback on the serial port about your actions.
For loading this on your fomu and connecting to it just run: make ctrl-load && screen /dev/ttyACM0